Barring someone finding an answer in the books, I think you could go two ways.
1 - Roll one critical roll with the weapon. If it's above an A crit (necessary to access the Large crit table) then apply that Large crit from the weapon crit table. Then apply the 1-degree lower elemental critical from Spell Law (or other tables, if you're using something else for elemental crits.)
2 - Roll one critical roll with the weapon. If it's above an A crit (necessary to access the Large crit table) then apply that Large crit from the weapon crit table. Then, if the 1-degree lower crit is above an A crit, it qualifies to deliver a Large spell crit on the Spell Law tables (table 15.8.5).
I know there's something about dual crits using the same roll, but I don't have that handy so I'll leave that part to someone else.